Prior to the present invention as shown by Ryang U.S. Pat. No. 4,381,396 assigned to the same assignee as the present invention, silylnorbornane anhydrides were made by reacting 5-norbornene-2,3-dicarboxylic anhydride or "norbornene anhydride" with a silane such as dimethylchlorosilane and an organic solvent, for example, toluene in the presence of a platinum catalyst. The mixture was then subjected to a vacuum stripping operation to remove volatiles. The resulting diorganohalosilylnorbornane 2,3-dicarboxylic anhydride, such as 5-dimethylchlorosilyl-norbornane-2,3-dicarboxylic anhydride was then hydrolyzed in water at 0.degree. C. in an organic solvent, such as tetrahydrofuran. The hydrolysis product was then stripped by heating to an elevated temperature, such as 200.degree. C. under reduced pressure. There was obtained an 80% yield of 5,5'-(1,1,3,3-tetramethyl-1,1,1,3-disiloxanedialyl)-bis-norbornane-2,3-dic arboxylic anhydride having the formula ##STR1## referred to hereinafter as "siloxane norbornane bisanhydride".
The present invention is based on discovery that siloxanenorbornane bisanhydride can be made by initially effecting a reaction between norbornene anhydride with dimethylchlorosilane in the presence of an effective amount of platinum catalyst and an organic solvent, followed by the vacuum distillation of the resulting mixture to produce 5-dimethylchlorosilyl-norbornane 2,3-dicarboxylic anhydride. Unexpectedly, the hydrolysis of this chlorosilyl-norbornane anhydride at a temperature of about 100.degree.-180.degree. C., while in the molten state, and its stripping under reduced pressure, has been found to provide siloxanenorbornane bisanhydride at yields exceeding 95%, such as 98%. Furthermore, the hydrolysis of the acid anhydride, a well known reaction, is not observed.